osx: implement media keys

Media keys are pretty handy if you use mpv as a music player (yes I'm one of
those people that do).

These are the bindings (which lead to the same behaviour as iTunes):
  * NX_KEYTYPE_PLAY   -> MP_KEY_PLAY
  * NX_KEYTYPE_FAST   -> MP_KEY_NEXT
  * NX_KEYTYPE_REWIND -> MP_KEY_PREV

I just handled these ones as the volume one would be pretty invasive. I could
maybe change it to increase the application's volume instead of system volume
only when mpv is frontmost (iTunes does this), but some users would probably
hate it.

osdep/macosx_application.m

@@ -120,7 +120,19 @@ static NSString *escape_loadfile_name(NSString *input)
 
 - (void)sendEvent:(NSEvent *)event
 {
-    [super sendEvent:event];
+    if ([event type] == NSSystemDefined && [event subtype] == 8) {
+        // It's a media key! Handle it specially. The magic numbers are reverse
+        // engineered and found on several blog posts. Unfortunately there is
+        // no public API for this. F-bomb.
+        int code  = (([event data1] & 0xFFFF0000) >> 16);
+        int flags = ([event data1] & 0x0000FFFF);
+        int down  = (((flags & 0xFF00) >> 8)) == 0xA;
+
+        if (down)
+            [self.eventsResponder handleMediaKey:code];
+    } else {
+        [super sendEvent:event];
+    }
 
     if (self.inputContext)
         mp_input_wakeup(self.inputContext);

osdep/macosx_application_objc.h

@@ -27,6 +27,7 @@ struct cocoa_input_queue;
 @end
 
 @interface EventsResponder : NSResponder
+- (void)handleMediaKey:(int)key;
 - (NSEvent *)handleKeyDown:(NSEvent *)event;
 @end
 

osdep/macosx_events.m

@@ -20,6 +20,9 @@
 // Carbon header is included but Carbon is NOT linked to mpv's binary. This
 // file only needs this include to use the keycode definitions in keymap.
 #import <Carbon/Carbon.h>
+
+// Media keys definitions
+#import <IOKit/hidsystem/ev_keymap.h>
 #import <Cocoa/Cocoa.h>
 
 #include "talloc.h"
@@ -107,6 +110,22 @@ void cocoa_put_key(int keycode)
 {
     return @[@"h", @"q", @"Q", @"0", @"1", @"2"];
 }
+- (void)handleMediaKey:(int)key
+{
+    switch (key) {
+        case NX_KEYTYPE_PLAY:
+            cocoa_put_key(MP_KEY_PLAY);
+            break;
+
+        case NX_KEYTYPE_FAST:
+            cocoa_put_key(MP_KEY_NEXT);
+            break;
+
+        case NX_KEYTYPE_REWIND:
+            cocoa_put_key(MP_KEY_PREV);
+            break;
+    }
+}
 - (NSEvent*)handleKeyDown:(NSEvent *)event
 {
     NSString *chars;